You can put the cartridge back if you want to. If it is empty you can either buy a new one and replace as needed or I have seen bottles of ink that refill old cartridges. I dont recommend the later one. I have heard that it is very difficult to refill and the ink quality isn't as nice. Welcome the expensive world of computers. Good luck.
p.s. I usually go to costco and buy bulk, saves a lot of $$$$.